In general, a rocket nozzle includes three sections: a converging section, a diverging section, and a throat section positioned between the converging and diverging sections. Combustion products from a rocket motor enter the rocket nozzle at the converging section, generating high pressure, high temperature gases that flow out of the converging section via the throat section. The throat section is a constricted region that forces the gas to accelerate as it enters the diverging section.
As the gas passes through the diverging section, the pressure and temperature of the gas decreases, but the velocity of the gas greatly increases. The velocity of the gas at the exit of the diverging section, or exhaust velocity, multiplied by the rate at which mass is ejected from the diverging section, or mass flow rate, equals a portion of the overall thrust of the rocket, namely the momentum thrust, which propels the rocket. Depending on the design of the nozzle and the altitude at which the rocket is operating, the overall thrust of the rocket may further include a thrust component referred to as pressure thrust. Pressure thrust is a function of the area of the nozzle exit and the difference in pressure between the gas at the nozzle exit and the atmospheric pressure.
Rockets often include fins, which may be engaged to the rocket nozzle. The fins increase the stability of the rocket and provide lift to increase the range of the rocket.